Generally, in driver circuits for flash tubes, it is desirable to control the amount of energy provided to a flash tube connected to the driver circuit as well as the color temperature of the resulting emitted light from the flash tube.
A driver circuit typically comprises at capacitor C configured to feed energy to a flash tube for a flash. The flash tube discharge by igniting ignition circuits inside the flash tube and thus drains the capacitor C. A first method of controlling the amount of energy provided to a flash tube and the color temperature of the emitted light from the flash tube is illustrated in FIGS. 1A-1B. In FIG. 1A, by charging the capacitor C up to a particular charging voltage, an amount of energy corresponding to the energy level EC is stored in the capacitor C. When said amount of energy EC is provided to the flash tube, the resulting emitted light from the flash tube will have the desired color temperature Tdes. If the capacitor C is instead charged up to a lower charging voltage, a lower amount of energy corresponding to the energy level Edes is stored in the capacitor C. Thus, when said lower amount of energy Edes is provided to the flash device, the resulting emitted light from the flash device will instead have the color temperature TB. However, it may often be desirable to achieve the desired color temperature Tdes of the resulting emitted light from the flash device, but while only providing the amount of energy Edes to the flash device.
In FIG. 1B, the capacitor C is charged to a particular charging voltage V corresponding to an amount of energy Edes+E′. As the amount of energy in the capacitor C is drained by the flash device, the discharge of energy is interrupted at time t1 when the amount of already discharged energy by the flash device corresponds to the desired amount of energy Edes. This will result in that the remaining amount of energy E′ is cut off and not discharged by the flash device. Consequently, the emitted light from the flash tube will have the color temperature T1. According to the inherent relationships shown in FIG. 1B, a particular charging voltage V and a discharge interruption timing t1 can be found such that the amount of energy provided to the flash tube is Edes and the color temperature T1 is approximately the same as Tdes, i.e. T1≈Tdes. Thus, in case of using a flash tube, it is in this manner possible to provide a desired amount of energy Edes to the flash tube and still achieve the desired color temperature Tdes of the resulting emitted light, as shown by the arrow in FIG. 1A.
A second method of controlling the amount of energy provided to a flash tube and the color temperature of the emitted light from the flash tube is to have a set or bank of different capacitors, e.g. C1-C3, which are configured to provide energy to the flash tube for the flash. This is illustrated in FIGS. 2A-2B. A given capacitor, e.g. C3, of a particular capacitance being charged to a particular charging voltage V3 corresponding to an energy level E3 will generate a particular color temperature Tdes of the emitted light when provided to a flash device at a flash tube. Here, if a different amount of energy is desired to be provided to the flash tube for the flash, while keeping the color temperature Tdes of the emitted light, any one of the different capacitors C1-C3 may be used separately or be combined to provide the desired amount of energy. However, since the number of capacitors sources C1-C3 in the set is finite due to the inherent implementation and economic considerations of having a large amount of capacitors, only finite number of discrete energy levels, e.g. E1, E2, E3, E1+E2, E1+E3, E2+E3, E1+E2+E3, will be possible for the desired color temperature Tdes.
However, both of the methods described above suffer from disadvantages. For example, by using the first method described above in reference to FIGS. 1A-1B, the amount of energy EC has to be lowered in order for the flash tube to get a desired color temperature. Another disadvantage with the first method is that the circuits used to interrupt the current have difficulties handle high currents.
Furthermore, achieving according to the second method a desired color temperature Tdes for a continuous, non-discrete range of energy levels E for even a flash device is not a scalable or cost efficient solution.
There is therefore a need for an improved solution for achieving a desired color temperature Tdes, which solution solves or at least mitigates at least one of the above mentioned problems.